Opportunity attacks are why you can’t just walk past the ogre. They make the front line matter and give melee characters a way to protect the people behind them.
What triggers one
You can make an opportunity attack when a hostile creature you can see moves out of your reach. You use your reaction to make one melee attack against it, just before it leaves. Reach is usually 5 feet, or 10 feet with a reach weapon like a glaive.
What doesn’t trigger one
- Moving around inside your reach, or moving toward you.
- Taking the Disengage action first (rogues and monks can do this as a bonus action).
- Teleporting, such as with misty step.
- Being moved by someone else — shoved, pulled or thrown — without using its own movement or reaction.
- Leaving the reach of a creature that can’t see you.
You only get one reaction
A creature gets one reaction per round, and it comes back at the start of its turn. If you use it on an opportunity attack, you can’t also cast shield or counterspell until your next turn. That trade-off is part of the fun for casters.
Tactics for both sides
- Fighters and paladins: stand between the enemy and your casters; feats like Sentinel make your opportunity attacks stop movement.
- Casters and archers: Disengage or teleport out instead of walking away from a sword.
- DMs: monsters that ignore opportunity attacks feel unfair. Let clever enemies Disengage too.
Keep track on the table
The initiative tracker shows whose reaction is spent each round, and the encounter builder helps you plan fights where positioning matters. Rules reference: Actions in Combat.